The present invention relates to a toner for developing an electrostatic latent image which is used to develop an electrostatic latent image in an electrophotographic method and an electrostatic recording method, and an image forming method and an image forming apparatus using the same.
In recent years, PC and networking have been rapidly widespread in offices, and the market of monochromic copying machines and printers which were mainly used in the past is currently being changed to the market of full-color ones. Consequently, the market of electrophotographic copying machines and printing machines which have been so far advantageous in view of an image quality and a speed has been increasingly demanded. Especially, not only a high image quality and high reliability but also downsizing, weight reduction, cost reduction and a high speed as well as the ecological measures such as energy saving, preservation of resources and recycling have been strongly required in the latest market. In order to meet the same, improvements and new developments of an image forming method and a developer used therein have been conducted.
An electrophotographic image forming apparatus (method) generally includes a charging unit (step) of uniformly charging a surface of an electrostatic latent image support, an exposure unit (step) of exposing the surface of the latent image support to form the electrostatic latent image, a developing unit (step) of developing the latent image on the surface of the electrostatic latent image support using a developer layer formed on a surface of a developer support to obtain a toner image, a transfer unit (step) of transferring the toner image on a transfer member, a fixing unit (step) of fixing the toner image on the transfer member, and a cleaning unit (step) of removing the toner remaining on the surface of the electrostatic latent image support in the transfer unit (step). A large number of basic characteristics required of a toner in these units (steps) are an appropriate amount of charge of a toner, charge duration and environmental stability in the developing unit (step), good transferability in the transfer unit (step), low-temperature fixability and offset resistance in the fixing unit (step), and cleaning performance and contamination resistance in the cleaning unit (step). In recent years, owing to the promotion of the high image quality, the high speed and the coloration, the characteristics which are more complicated have been in demand.
For example, in the transfer unit (step), an indirect transfer-type image forming apparatus in which for expediting registration in formation of a color image, toner images on a surface of an electrostatic latent image support are transferred and overlapped in order using an intermediate transfer member and then transferred at once on a transfer member is becoming a mainstream of full-color copying machines and printers in recent years because a higher speed and a higher image quality can be realized. Nevertheless, since the number of transfers of the toner is increased in this indirect transfer-type image forming apparatus, more accurate transferability is required for increasing the image quality, and an additive and a technique of controlling a shape and a surface structure of a toner for providing more stable chargeability and improving transferability have been required of the toner.
Further, with respect to the cleaning unit (step), from not only downsizing and cost reduction of an apparatus but also an ecological standpoint such as energy saving, reservation of resources and reduction of waste, the decrease in amount of a transfer residual toner and reduction in size or elimination (cleaner-less) of a cleaning unit are important subjects. Especially in a full-color image forming apparatus using toners of four colors, yellow, magenta, cyan and black, a transfer residual toner is a serious problem.
In order to avoid the new problems in the transfer and cleaning units (steps), it is important to minimize the amount of the residual toner. To this end, it is necessary to increase transfer efficiency of the toner. For increasing transfer efficiency, it is important to transfer toner particles directly adhered to an electrostatic latent image support, and it is effective to decrease adhesion between the toner and the electrostatic latent image support. With respect to such a method, a method is proposed in which separable fine particles of silica are incorporated in a developer and interposed between a toner and an electrostatic latent image support to decrease adhesion between the toner and the electrostatic latent image support and increase transfer efficiency of the toner as described in, for example, Japanese Patent Laid-Open Nos. 2-1870, 2-81053, 2-118671, 2-118672 and 2-157766. This method improves indeed the transfer efficiency of the toner from the electrostatic latent image support. However, in case of using the intermediate transfer member, no satisfactory transferability can be provided by only decreasing physical adhesion of the toner.
That is, since the toner is transferred with an electrostatic attraction force by applying bias having a reverse polarity to that of the toner to a transfer member, a toner component charged with a reverse polarity or zero-charged cannot be transferred on a transfer member. Further, so-called retransfer occurs in which a toner once transferred in overlapping color images is reversely transferred on an electrostatic latent image support or an intermediate transfer member. Accordingly, in order to maintain high transfer efficiency, it is important to decrease physical adhesion of a toner and to uniformly maintain a toner charge distribution before and after transfer.
As a flowing agent of a toner, hydrophobic silica obtained by a dry method has been so far used. However, since the hydrophobic silica obtained by the dry method has large environmental dependence in charging, a charge distribution tends to be broadened especially at a low temperature and low humidity, and a low-charged toner or a toner charged with a reverse polarity is present in a large amount. Therefore, the transfer deficiency tends to occur. For providing a sharp toner charge distribution, a method in which inorganic fine particles of titanium oxide having relatively low electrical resistance and good charge exchangeability are added to silica particles is known. The use of inorganic fine particles having low electrical resistance provides a narrow charge distribution and decreases an amount of a toner charged with a reverse polarity. However, a toner charge distribution on a transfer member is liable to change by charge injection in a transfer electric field, and the transfer deficiency also tends to occur.
Meanwhile, as another method of improving environmental dependence of the hydrophobic silica obtained by the dry method, a method in which negative silica is mixed with positive silica obtained by positive treatment, and a method in which negative silica is mixed with resin fine particles of PMMA having a positive polarity are proposed. These methods can improve indeed environmental dependence by controlling the increase in charge at a low temperature and low humidity. However, since a charge amount of the toner is decreased as a whole, this method is not satisfactory to improve the transfer deficiency owing to the low-charged toner or the toner charged with a reverse polarity as described above.
Further, as still another method for improving environmental dependence and a charge distribution of silica obtained by a dry method, Japanese Patent Laid-Open No. 4-80,764 proposes that silica obtained by a dry method and having hydrophobic property of 60 to 90% and silica obtained by a wet method and having hydrophobic property of 50 to 80% are used in combination. Compared with silica obtained by a dry method, silica obtained by a wet method has a more porous inner structure and a high adsorbed water content, providing a low level of charge. However, when it is used in combination with silica obtained by a dry method, a toner having a high level of charge and a sharp charge distribution can be obtained. Nevertheless, although silica obtained by a wet method has high apparent hydrophobic property by hydrophobic treatment, electrical resistance is relatively low, and a particle size is large as compared with silica obtained by a dry method. Accordingly, this silica tends to be a charge injection site on a surface of a toner, and the change in charge by a transfer electric field is liable to occur. Especially, silica obtained by decomposing sodium silicate with an acid or an alkali metal salt by a wet method tends to have lower electrical resistance owing to an influence of an alkali ion component remaining therein, and it is difficult to obtain stable transferability.
The invention has been made in view of these circumstances, and provides a toner for developing an electrostatic latent image, which is good in transferability over a long period of time and gives a high image quality without causing an image defect, and an image forming method and an image forming apparatus using the same.
According to an aspect of the invention, the toner for developing an electrostatic latent image includes toner particles containing a binder resin, a colorant, and an external additive, the external additive containing silica whose surface is treated to be hydrophobic and which has an average primary particle size of 80 to 300 nm, a water content of 3 to 15% and a volume resistivity of 1xc3x971013 xcexa9cm or more.
According to another aspect of the invention, the electrostatic latent image developer includes a carrier and a toner, the toner being the above-described toner.
According to another aspect of the invention, the image forming method includes a charging step of uniformly charging a surface of an electrostatic latent image bearing member, an exposure step of exposing the surface of the electrostatic latent image bearing member to form the electrostatic latent image, a developing step of developing the latent image on the surface of the electrostatic latent image bearing member using a developer layer containing a toner formed on a surface of a developer bearing member to obtain a toner image, a transfer step of transferring the toner image on a transfer member, and a fixing step of fixing the toner image on the transfer member, the toner being the foregoing toner.
According to another aspect of the invention, the image forming apparatus includes a charging unit that uniformly charges a surface of an electrostatic latent image bearing member, an exposure unit that exposes the surface of the electrostatic latent image bearing member to form the electrostatic latent image, a developing unit that develops the latent image on the surface of the electrostatic latent image bearing member using a toner-containing developer layer formed on a surface of a developer bearing member to obtain a toner image, a transfer unit that transfers the toner image on a transfer member, and a fixing unit that fixes the toner image on the transfer member, the toner being the foregoing toner.